The aim of this research project is to further the understanding of the fundamental electronic fine structure of heme. A better knowledge of heme's electronic structure should lead to a more complete understanding of how heme functions in its biological roles. In this work heme compounds in an order of systematically increasing complexity are being studied by the technique of electron nuclear double resonance (ENDOR). Initial research is being carried out on anionically liganded pure heme, free of protein, with subsequent studies on a heme-peptide model system. Finally work will be carried out on heme proteins (hemoglobin and myoglobin derivatives, peroxidase, and catalase) which have not yet been studied by ENDOR. The goals of this proposed work are: 1. To provide a systematic basis on which to understand the electronic structure of heme in proteins. 2. To provide correlations with the data from other physical techniques which have probed different aspects of heme's electronic structure. 3. To provide experimental input data for a comparison with theoretical predictions of quantum mechanics on heme.